1. Field of the Invention
This invention relates to an ink-jet printer and more in particular to improvements in the charge amount control type ink-jet printing apparatus capable of causing flying ink droplets to land at desired locations on recording paper thereby producing a printed character thereon without distortion.
2. Description of the Prior Art
In the charge amount control type ink-jet printing apparatus, flying ink droplets are charged individually in sequence in accordance with the information as to a character to be printed and the thus charged ink droplets are deflected selectively in a constant electric field formed between a pair of deflection plates to impinge upon recording paper at desired locations, thereby forming a desired imprint thereon. In this case, a local flow is induced in the wake of a flying ink droplet. Thus, when the immediately following ink droplet comes under the influence of such a local flow, its aerodynamic drag force is decreased so that the immediately following ink droplet comes closer to and eventually catches up with the preceding ink droplet to form an integrated ink droplet thereby causing print distortion.
Since all or some of the flying ink droplets are charged, they influence each other electrostatically under the Coulomb's law to cause the spacing between ink droplets irregular, resulting in production of print distortion. Moreover, as influenced by the preceding charged ink droplet, the next following ink droplet just about to be charged will be insufficiently charged. This can also be a cause of the production of print error.
One approach to cope with the above-mentioned problems is disclosed in the U.S. Pat. No. 3,946,399. That is, in accordance with the idea disclosed in the above-mentioned patent, in order to compensate for electrostatic effects between charged ink droplets as well as aerodynamic effects, a character pattern to be printed is detected in advance and the charging amount of an ink droplet is compensated in accordance with the detected charge pattern prior to the printing step. However, such a technique is of little value when the number of deflection steps is increased to about 32. Because, when the number of deflection steps increases, ink droplets are more narrowly spaced from each other when they fly so that the level of print distortion increases. Besides, since the flying time of each ink droplet differs depending upon the amount of deflection, the level of print distortion also varies depending upon the amount of deflection. Therefore, in order to carry out a proper compensation to avoid print distortion, such a compensation must be adjustable for the number of deflection steps.
The ink-jet printer system shown in FIGS. 1 through 3 is so structured that it may carry out a proper compensation to avoid print distortion in accordance with the contents of print data and the number of deflection steps. That is, the ink-jet printer shown in FIG. 1 comprises a clock pulse generator 1 having its output connected to an input of a frequency divider 2. The frequency divider 2 supplies a first frequency output f.sub.1 to a phase shifter 3 which supplies its output to an ink-jet discharging head through an amplifier 4. The phase shifter 3 is also connected from a charge phase detecting electrode through an amplifier 5. The output of the clock pulse generator 1 is also connected to the phase shifter 3 and to a check pulse generator 6, which, in turn, is connected to an amplifier 9. A charge compensating circuit 7 is connected to the frequency divider 2 in such a manner to receive second and third frequency outputs f.sub.2 and f.sub.3. The charge compensating circuit 7 also receives print or character data to be printed and a print command signal and supplies its output to a digital-to-analog (D/A) converter, which, in turn, supplies its analog output to the amplifier 9 connected to a charging electrode for charging each ink droplet to be charged.
In one embodiment of the system shown in FIG. 1, the frequency of a clock pulse signal generated from the clock pulse generator 1 is 1,056 kHz and when such a clock pulse signal is supplied to the frequency divider 2, an exciting pulse signal f.sub.1 having the frequency of 132 kHz, a charging pulse signal f.sub.2 having the frequency of 44 kHz and a compensating pulse signal f.sub.3 having the frequency of 352 kHz are produced. It is to be noted that the exciting pulse signal of 132 kHz is applied to the ink-jet discharging head and two uncharged guard droplets between charged droplets are provided thereby decreasing the level of print distortion. In other words, every three droplets are charged in this embodiment. And thus the frequency of charging ink droplets is 44 kHz. If desired, however, such guard droplets may be eliminated. The charging voltage may be varied in the range between 50 V. and 240 V.
The charge compensating circuit 7 stores a number of compensating amounts in the form of charging codes, and an arithmetic operation of addition or non-addition of compensating amounts to a reference charging value is controlled in accordance with the presence or absence of a print or character data as the stored compensating amounts are read out sequentially. The resulting added quantity is converted into an analog signal by the D/A converter 8 and then applied to the charging electrode after being amplified by the amplifier 9. The compensating amounts are stored in the form of the binary number in a memory such as a read only memory (ROM) or a random access memory (RAM) provided in the charge compensating circuit 7. Together with these compensating amounts, reference charge amounts also in the form of codes, which are so formed to allow respective ink droplets to reach predetermined locations when they are deflected one by one as individual droplets, are also stored in the memory.
FIG. 2 shows the compensating and reference charge amounts expressed in the form of predetermined codes in a tabulated form. It is to be noted that each code in the table shown in FIG. 2 is, in reality, comprised of 11 bits; however, in the table shown, each code of 11 bits is represented as a three digit code by converting the first three bits into an octal number and the second four bits and the remaining four bits into hexadecimal numbers. Accordingly, the memory should have an 11-bit parallel structure and the capacity of the bit number determined by 11(bits).times.8(dots).times.32(steps) or more. Incidentally, the code "7FF" at step 32 in column F.sub.2 is to add a complement so as to carry out the arithmetic operation of adding the negative number -1. As may be noticed, the reference charge codes in column V.sub.cs are non-linear since the aerodynamic drag force varies depending upon the amount of deflection. The compensating amounts are determined on the basis of individual ink droplets; however, in determining the reference charge amounts and compensating amounts, their tentative values are first obtained by a computer simulation and then these tentative values are modified by empirical data. In the embodiment shown in FIG. 2, there are 32 deflection steps and a compensation may be carried out for the four preceding or leading droplets P.sub.1 through P.sub.4 and the three following or trailing droplets F.sub.1 through F.sub.3 for a particular droplet V.sub.cs. In other words, for a particular droplet, the effects due to the preceding four and following three droplets may be compensated by adding the corresponding compensating amounts to the reference amount V.sub.cs.
FIGS. 3a and 3b taken together as indicated in FIG. 3 show the detailed structure of the charge compensating circuit 7 which includes an address counter 11 which receives a print command signal and a compensating clock pulse and supplies its output to a memory 12 such as a ROM. The output from the memory 12 is supplied to an adder 14 through a gate 13. The circuit 7 also includes a shift register 15 which receives a charging pulse signal and character data and supplies its output to a multiplexer 16 which also receives an input from the address counter 11. The output of the multiplexer 16 is connected to an input of the gate 13. Also provided are a latch 17, a D/Q flipflop 18 and a gate 19 as connected as shown.
In operation, when the print command signal goes high, the address counter 11 becomes operative and starts counting operation in association with the compensating pulse signal f.sub.3. As is obvious, since the compensating pulse signal f.sub.3 has the frequency eight times higher than that of the charging pulse signal f.sub.2, eight data for a particular deflection step arranged in a row in the table shown in FIG. 2 are read out for each cycle of the charging pulse signal. Print or character data, comprised of a series of the binary numbers with "1" indicating the presence of a print dot or picture element and "0" indicating the absence of a print dot, is shifted into the shift register 15. The output supplied from the shift register 15 is an eight bit parallel output comprised of O.sub.0 -O.sub.7, in which O.sub.3 corresponds to the reference charging data to be printed with O.sub.0, O.sub.1 and O.sub.2 carrying information as to the presence or absence of a print dot for the following three ink droplets, which corresponds to F.sub.3, F.sub.2 and F.sub.1, respectively, in the table of FIG. 2, and O.sub.4 through O.sub.7 carrying information for the preceding ink droplets and corresponding to P.sub.1 through P.sub.4, respectively, in the table.
The lower three bits of the output from the address counter 11 are introduced into the multiplexer 16. When these lower three bits are all "0", the multiplexer 16 allows to supply the contents of O.sub.0 as its output; whereas, when the lower three bits are all "1", the contents of O.sub.1 are supplied as an output. In other words, seven data in front and rear of the print data, or eight data in total are selectively supplied as an output one by one depending upon the contents of the lower three bits of an output from the address counter 11.
An output from the multiplexer 16 is applied to the gate circuit 13 which controls the supply of an output from the memory 12. That is, when an output from the multiplexer 16 is high thereby indicating "compensation required", the output from the memory 12 is supplied to the adder 14 through the gate circuit 13 to be added as a compensating amount. An output from the adder 14 is then latched into the latch circuit 17 and then added accumulatively to the following compensating amount when it is supplied to the adder 14. However, when the contents of the lower three bits of an output from the address counter 11 are "7", the input to the latch 17 is inhibited thereby resetting the contents of the latch 17 to "0."
An output from the adder 14 is also supplied to the D/Q flipflop 18 and sampling is carried out at the rising edge of a charging pulse. In this manner, the compensated charging value is now stored in the D/Q flipflop 18 and its fate is controlled by the presence or absence of print data. That is, when the present print data is indicated, the compensated value now stored in the flipflop 18 is supplied through the gate circuit 19 to the D/A converter 8 as a charging code thereby allowing to carry out a required compensation.
As described above, in the ink-jet printer system shown in FIGS. 1 through 3, it is true that a compensation for the interaction with the other ink droplets, in particular charged droplets, to avoid print distortion may be carried out. However, such a compensation alone is not always sufficient in the ink-jet printing technology. That is, even for a particular deflection step, there could be a deflection error due to disturbances in reference conditions other than interaction with the other droplets such as temperature and moisture. If temperature changes to vary the viscosity of the ink, then such a change can cause a deflection error even under the circumstances where no other ink droplets are present. Thus, in order to carry out a total compensation, such a deflection error must be corrected as well as a compensation for the interactive effects with the other droplets.
However, if both of deflection error and interactive error compensations were to be carried out digitally, it would require a large capacity memory and, moreover, there would be a significant delay in calculating the required compensating values. As a result, real-time processing cannot be carried out and the overall structure tends to be complicated, which are disadvantageous.
Stated more in detail, in the charge amount control type ink-jet printer as described above, nth step deflection in reference conditions is given by applying a charging voltage V.sub.cn ', which takes into account interactive effects with other droplets, to the charging electrode as expressed in the following equation. EQU V.sub.cn '=.SIGMA.P.sub.n-k .multidot.V.sub.cs(n-k) .multidot.C.sub.n-k +V.sub.csn +.SIGMA.F.sub.n+k .multidot.V.sub.cs(n+k) C.sub.n+k ( 1)
where, P.sub.n-k : distortion rate of the preceding droplet; F.sub.n+k : distortion rate of the following droplet; V.sub.cs(n.+-.k) : reference code of (n.+-.k)th step single droplet; and C.sub.n.+-.k : presence or absence of (n.+-.k)th step droplet.
On the other hand, in detecting and controlling the amount of deflection of an ink droplet, use is made of, for example, a 32nd step single droplet, which is deflected to carry out the required detection and control. In this case, assuming that the current charging voltage code is V.sub.xd, the nth step charging voltage V.sub.cn will be obtained as shown by the following equation. ##EQU1## When a compensation is to be carried out in accordance with the above equation (2), the value of V.sub.xd is first determined by increasing or decreasing the value of the charging voltage code in response to an output from the defleciton amount detecting mechanism, and, then, upon completion of detection and control of the amount of deflection, an arithmetic operation is carried out for the factor ##EQU2## thereby determining the compensation rate. Then, upon initiation of printing operation, the amount of distortion compensation expressed by the above equation (1) is calculated and it is multiplied by the above-obtained compensation rate. It will thus be understood that a high speed adder is required in the above described system.